1. Field of the Invention
The present invention relates to an improved drive mechanism adapted for use with a harmonic drive transmission to form a drive assembly, particularly useful for powering the controlled infeed or controlled outfeed of a printing press or the like for maintaining proper web tension in the printing press. More particularly, it is concerned with such a drive mechanism which interfaces between the power input shaft and wave generator tuner shaft of the harmonic drive transmission in such a manner that the rotational velocity of the wave generator is precisely prescribed.
2. Description of the Prior Art
In the printing press industry, it is quite typical to employ several work stations to perform a variety of work functions on a continuous web of paper travelling through the printing press. Thus, such a printing press might employ several lithographic printing stations, a dryer through which the web presses after being printed, a chill roll station for cooling the heated paper web after exiting the dryer, and possibly a bindery station for performing a cutting, sheeting, stitching, etc. bindery function on the moving web.
Those skilled in the art will appreciate that in such a web fed printing press, it is necessary to maintain the web under proper tension while travelling through the printing press. Thus, it is desirable to feed the web into printing stations through use of a controlled infeed so that proper tension is maintained in the web traveling through the printing stations. Proper web tension is necessary in the printing stations to give acceptable print quality and proper color-to-color registration. This web tension can be achieved by, for example, operating the controlled infeed at a speed of -1.0% to 0.0% relative to the operating speed of the printing units.
Typically, in such printing presses the web is maintained under tension while transiting the dryer. That is, the web would be allowed to follow a "sine" wave path through the dryer to achieve more efficient drying of the web substrate and disrupt any boundary layer effect contiguous the web. A chill unit is typically employed following the dryer to cool the heated paper web and to provide an anchor point for the web for further printing press operation. For example, if a sheeting mechanism is employed in the printing press downstream of the chill stand, it is necessary to feed the web under proper tension to the sheeter to achieve proper cutoff lengths of the sheets. The sheet cutoff length is determined in large part by the web tension between the chill stand and the sheeter. Too high a tension in the web often results in a short cutoff sheet length, while too low a web tension usually results in too long of a cutoff sheet length. To achieve the proper web path through the dryer and the proper web tension after exiting the chill stand, it is desirable to have the capability to operate the chill stand rollers in a speed range from slightly less to slightly greater than the operating speed of the printing stations (e.g. -0.5% to +1.5%).
To achieve proper web tension in a printing press, drive transmissions have been employed for operating the controlled infeed and chill stands (or other printing press operating units) using a variable speed transmission. It has been found that such variable speed transmissions operate best by using so-called harmonic drive transmissions to power the controlled infeed or chill stand. Harmonic gearing utilized in such harmonic drive transmissions is described in U.S. Pat. No. 2,906,143 to C. W. Musser, U.S. Pat. No. 3,565,006 to Stewart, and advertising brochures distributed by the United Shoe Machinery Corporation entitled "Harmonic Drive Pancake Gearing" HDUF13000-76 and "Harmonic Drive" HD-1000-5M-12/75, all of which are herein incorporated by reference.
Generally speaking, a harmonic drive gear includes a rotatable, elliptical wave generator; a flexible, externally-toothed, rotatable spline disposed about the wave generator; and an internally-toothed, circular spline disposed about the flexible spline. Preferably, the circular spline has a greater number of teeth than the flexible spline (e.g. two teeth), determined largely by the reduction ratio desired. In operation, rotation of the wave generator imparts a rotating elliptical shape to the flexible spline causing progressive engagement of its external teeth with the internal teeth of the circular spline, thereby imparting relative rotation to the flexible spline. It will be appreciated that the three basic harmonic drive components can be employed in a variety of ways to accomplish different power transmission functions, as more fully explained in the above-referenced patents and brochures.
It has heretofore been suggested to rotate the wave generator of such a harmonic drive transmission in such a way that the rotational velocity of the wave generator can be varied as desired. To this end, various drive mechanism have heretofore been employed between an input drive shaft and the wave generator or tuner drive shaft such that the rotational speed of the tuner shaft can be controlled. For example, one such drive mechanism includes a variable sheave pulley secured to the tuner drive shaft and a pivotal arm mounted for pivotal movement about the input drive shaft. The pivotal arm includes one or more pulleys drivingly interconnected to the input shaft. One of the pulleys on the pivotal arm is connected by a V-belt to the variable sheave pulley on the tuner shaft such that pivoting of the arm varies the rotational speed of the variable sheave pulley and in turn the tuner shaft.
Several problems, however, exist with this past drive mechanism design. For example, it was discovered that the speed range desirable for input to a controlled infeed or chill stand of a printing press could not be achieved using such a mechanism. Further, this mechanism was not only costly, but in addition, had stability problems with the pivotal arm. Thus, a significant advance in the art would be realized if an inexpensive drive mechanism were devised for interface with a harmonic drive transmission which had the speed range desired for use with the controlled infeed or chill stand of a printing press and did not exhibit the stability problems associated with past devices.